Weakened concrete joint and method of forming same



G. S. LACY Feb. 15, 1966 WEAKENED CONCRETE JOINT AND METHOD OF FORMINGSAME Filed June 4, 1962 INVENTOR. 60/900 6. 4146' 1 United States PatentO WEAKENED CON CRETE JOlNT AND METHUD F FORMING SAME Gordon S. Lacy,Escondido, Califi, assignor to Clardon Investment Co., Escondido, Calif,a partnership of California Filed June 4, 1962, Ser. No. 199,694 11Claims. (Cl. 94-18) This invention relates to poured concrete slabs andwalls and more particularly to the provision of a weakened joint of anew and improved character at predetermined spaced intervals and sodesigned that fracture of the slab due to shrinkage or loading occurssubstantially in the plane of the weakened joint and along irregularinterlocking surfaces.

It is well known that large area slabs and walls of poured concrete aresubject to shrinkage during curing of the cement, as well as toexpansion and contraction due to temperature changes. For these reasonsit is highly desirable that large expanses of concrete be provided withsuitable provision at spaced intervals to accommodate relative movementbetween adjacent sections of the concerete to compensate or allow forshrinkage, temperature changes and the like variable factors. Variousexpedients have been resorted to heretofore in dealing with theseproblems. For example in the construction of concrete pavements androadways and the like, it has been customary to pour individual slabs inend to end relation but separated from one another by an expansion jointof various types. One well known and widely used type of expansion jointcomprises a laminated strip of fibrous strips impregnated with tar andextending for the full depth and width of the slab. This and relatedprior expansion joints are costly and time-consuming to install andleave the adjacent slabs totally disconnected from one another for allpractical purposes. In consequence the application of a load to a cornerarea of a slab when the underlying ground or support is wet or softoften results in the application of severe tensile stresses on thecorner of the slab causing it to break off.

To obviate this serious shortcoming it has been proposed to pour theconcrete in continuous slabs and to cut a deep slot transversely of theslabs during a particular stage of the curing operation at spacedintervals using power driven diamond-tooth saws. This practice is quiteeffective in providing a weakened joint but is extremely time-consumingand costly. It is also found that the slots left by the saw cuttingtechnique tend to become filled with foreign material or water. Thismaterial becomes thoroughly packed in the slots seriously interferingwith the expansion of the concrete under high temperature conditions andthe freezing of the water can introduce severe destructive forces.

By the present invention there is provided a new and improved weakenedjoint for concrete slabs and wall structures of all types. This jointfeatures a narrow slot preferably filled by a continuous and permanentstrip of inexpensive resilient material as for example a soft andresilient plastic of either a homogeneous or spongy character and of asuitable cross-sectional shape. One edge of this slot opens through onesurface of the concrete, the slots customarily being generally paralleland appropriately spaced from one another in accordance withconventional practice. The depth of the slot is somewhat less thanone-half the thickness of the slab structure and sufficient to assureoccurrence of a fracture in a transverse area including the slotted arearather than in a portion of the slab to either side of the slot.

The cross-sectional shape of the material used to cast 3,234,860Patented Feb. 15, 1966 the slot may take various forms, such as a simplenarrow rectangle, or an inverted T with the stem or major dimension ofthe slot extending generally normal to and opening through one surfaceof the slab. A slot forming strip of inverted T-shape has the advantagethat the failure crevice may occure along a wider zone in that it may belocated in a vertical plane substantially anywhere between the oppositeends of the T-head. Hence it will be recognized that the failure crevicemay occur in a generally zig-zag fashion crosswise of the slab therebyfurther increasing the strength of the interlock between the juxtaposedirregular faces of the crevice. This provides added assurance thatadjacent slabs will share imposed loads equitably.

Irrespective of the cross-sectional shape of the slot forming strip, itmay be desirable to provide the web of the material with suspensionextension intended to extend beyond the finished surface of the slab tobe formed and having a weakened line opposite the surface of thefinished slab facilitating severing of the protruding portion of thestrip following setting of the concrete. Alternatively and if desired,the strip of slot forming material may be withdrawn from the slab afterthe concrete has taken a partial set, although in a preferred mode ofuse, the material is left in the slot permanently for the purpose ofexcluding liquid and foreign material and accommodating by itsresiliency changes in the width of the slot due to shrinkage andtemperature changes.

The slot forming material or strip may be supported in various waysduring pouring and setting of the concrete. If the slabs are notexcessively wide, the strips may be supported on rigid members extendingcrosswise of the slabs and suitably supported at their ends. In othersituations where the slabs are too Wide to permit of this expedient,stakes may be set at intervals crosswise of the slab and used to supportrigid members which in turn support the slot-forming strips at intervalsthere along, after the concrete has taken a partial set, the rigidmembers together with the stakes are withdrawn and the openings left bythe stakes are then filled with concrete mix flush with the exteriorsurface of the slab.

Accordingly it is a primary object of the present invention to providean improved inexpensive and highly effective weakened joint for concreteas well as a method of forming a weakened joint as an incident to thepouring of the concrete mix.

Another object of the invention is the provision of weakened jointsextending at spaced intervals crosswise of concrete slabs, walls and thelike, and so constructed that one face portion of the slab as initiallycast comprises continuous concrete whereas the other face portion istraversed by a deep narrow slot either left vacant or filled withresilient material capable of excluding foreign matter.

Another object of the invention is the provision of a weakened joint forconcrete slabs extending crosswise of the slab and extending partiallythrough the slab there- 'by forming a weakened cross-sectional area ofconcrete thereby to assure failure of the slab in this weakened zone andalong an irregular surface effective to form a high strength interlockbetween the adjacent faces of the failure crevice.

These and other more specific objects will appear upon reading thefollowing specification and claims and upon considering in connectiontherewith the attached drawing to which they relate.

Referring now to the drawing in which preferred embodiments of theinvention are illustrated.

FIGURE 1 is a fragmentary perspective view of a short length of aconcrete slab or wall showing a weakened joint incorporating thecharacteristic principles of this invention and in the process of beingformed;

FIGURE 2 is an enlarged fragmentary transverse view through the weakenedjoint of FIGURE 1 and indicating a likely shape of the failure crevicelikely to form in the slab;

FIGURE 3 is a fragmentary perspective view of an alternate mode offorming the weakened joint and particularly useful in slabs of widerwidths;

FIGURE 4 is a transverse cross-sectional view of the joint shown inFIGURE 3;

FIGURE 5 is a cross-sectional view of a third embodiment of the weakenedjoint according to the present invention; and

FIGURE 6 is a fragmentary sectional view on an enlarged scale throughthe plastic strip shown in FIGURES 1 to 4.

Referring more particularly to FIGURES 1 and 2, there is shown by way ofexample a slab of concrete 10 poured along the ground and suitable foruse as a drive-way and having a weakened concrete joint designatedgenerally 11 extending crosswise thereof. This weakened joint includes anarrow but deep slot 12 extending across the entire width of the slab.The depth of slot 12 may vary over a considerable range but excellentresults are achieved using a depth less than half but at least onefourth the thickness of the slab. This slot is formed during pouring ofthe concrete by the lower portion of a fiat strip 13 of suitableresilient material, secured as by nails 14 to a rigid supporting member15. Strip 13 may be formed from various resilient materials having ahomogeneous or cellular structure immune to attack by pavement chemicalsand by moisture. Such materials include various plastics and suchcellular materials as those formed from polystyrene, urethane andothers. If made of a spongy plastic it is desirable that the cells be ofthe closed type to exclude moisture and other fluids.

Desirably a deep groove 16 of either a continuous or intermittentcharacter is formed in one or both surfaces of member 13 and this ispositioned to lie flush with the finished surface of the slab tofacilitate severing of the portion above the groove and serving as meansfor suspending the strip proper from rigid member 15. As has beenpointed out above, strip 13 is secured to the lower edge of mountingmember 15 with groove 16 lying in the plane of the lower edge of member15 as is clearly illustrated in FIGURE 2.

Before pouring the concrete mix, member 15 is suitably supported at itsends by stakes or by forms for the concrete, not shown, but understoodas arranged along either lateral edge of slab 10. The slot formingmaterial 13 then lies suspended in a vertical plane so that, uponpouring of the concrete mix, the lower portion of the strip will projectdownwardly into the mix and form a slot 12 of the desired depth.

After the poured concrete has taken a partial set or even after it hastaken a complete set, member 15 together with the portion of the slotforming material 13 above severance groove 16 is torn or cut away flushwith the top surface 18 of the slab. Or, if desired, all of the slotforming material may be withdrawn from slot 12 soon after the concretehas been poured and after it has taken a sufficient set as not to slumpand fill the vacated slot. It will be understood that this intervalbetween weakened joints 11 may vary over a considerable range dependingon operating conditions the range of temperature changes duringdifferent periods of the year and other factors.

Under operating stresses or due to shrinkage or temperature changesposing severe strain on the completed slab, owing to the presence ofslot 12, the failure of crevice 20 is certain to occur between the loweredge of slot 12 and the bottom surface of slab 10. Such failureinvariably occurs along a highly irregular path, both ver- 'tically andhorizontally, thereby providing a multiplicity of interlocks with theresult that the juxtaposed faces of the crevice remain positivelyinterlocked with one are other. It follows that the passage of a vehicleor other load across surface 18 of the slab is ineffective to depressthe slab on one side of crevice 24 more than on the opposite side.Accordingly it will be understood that each of the adjacent sections ofthe slab will be constrained to share the load substantially equitablyand that the load is transmitted crosswise of crevice 20 by theinterlocking faces thereof. Owing to the fact that slot 12 is filledwith the resilient material 13, dirt, liquid, and other foreign mattercannot enter the slot.

Referring now to FIGURES 3 and 4, there is shown an alternate mode ofsupporting the slot filling material 13. It will be observed that therigid member 15 described in connection with FIGURES l and 2 is replacedby an angle iron 15' suitably supported at intervals therealong by anappropriate number of stakes 25 only one of which is here illustrated.Spaced at intervals along the downwardly projecting flange of member 15and suitably fixed thereto are brackets 26 having resilient springfingers 27 spaced apart suitably to receive and firmly grip the slotforming strip 13 astride its upper lateral edge. If desired one of thefingers 27 may be provided with an adjustable thumb nut or screw 28Operable when rotated to adjust one of the fingers toward the other toprovide a firmer grip on the strip 13. In the arrangement justdescribed, slot forming strip 13' need be no wider than the depth ofslots 12 thereby avoiding the need for wasting any of the strip materialor for providing either the severance groove 16 or a suspension portion.customarily stakes 25 supporting rigid member 15' are driven to a properdepth with the lower ends of the strip supporting members 26 lying flushwith the con templated finished surface of slab 10. Thereafter strip 13is pressed upwardly between the gripping fingers 27 until the upper edgeof the strips contacts the lower ends of support brackets 26. Thereafterthe concrete mix is poured in the usual manner. After the concrete hastaken a partial set, the opposite ends of members 15' are elevated alongwith stakes 25. Since the frictional grip of the concrete with strip 13'is stronger than the gripping action of fingers 27 therewith, strip 13'remains firmly in place in slots 12 as the stakes and members 15 arewithdrawn. Thereafter the holes left by stakes 25 are filled with mix.

Referring to FIGURE 5 an alternative design of the slot forming material13" is shown in its finished installed condition. Strip 13" is ofinverted T-shape in cross-section and may be formed of the same materialused in forming strips 13, 13' discussed above in connec tion withFIGURES l to 4. The length of the head portion 29 of member 13", as hereshown, corresponds generally with the length of stem portion 30.However, these relative dimensions will be understood as non-criticaland subject to wide variation in practice. The illustratedcross-sectional shape has the advantage of greatly increasing thestrength and handling capabilities of the strip during the installationand pouring operation. Additionally the length of the head portionextending generally parallel to the upper and lower surfaces of the slab10 tends to increase the positive interlocking character of the crevice20' customarily forming in the weakened section of the slab between thelower surface of member 13 and the bottom of the slab. This is due inmajor part to the likelihood that crevice 20' can develop in any portionof the concrete underlying head 29 of the strip 13" as: well as in amuch larger zig-zag path lengthwise of the. strip.

Although the weakened joint of this invention has beendescribed inconnection with slabs poured on the ground. it will be understood thatit is equally applicable to con-- crete slabs disposed in various planesand used in poured concrete constructions of all kinds includingvertical,

horizontal and inclined Walls. If for any reason it is desirable thatthe slot forming material be eliminated after the concrete takes a setand particularly in applications where the described weakened joint isemployed in wall structures, this may be done by either withdrawing thestrips before the concrete sets or by applying a suitable solvent to thestrip material. The solvent is applied and-left in contact with thematerial until softened sulficiently to permit the dissolved material tobe flushed away using strong jets of water or the like. If polystyrenefoam is used for the strip material, a solvent of the ketone classprovides very satisfactory results economically. Acetone is particularlysatisfactory and readily converts the polystyrene to a gas.

While the particular weakened concrete joint and method of forming sameherein shown and disclosed in detail is fully capable of attaining theobjects and providing the advantages hereinbefore stated, it is to beunderstood that it is merely illustrative of the presently preferredembodiments of the invention and that no limitations are intended to thedetails of construction or design herein shown other than as defined inthe appended claims.

I claim:

1. An article of manufacture comprising a strip of homogeneous plasticmaterial of uniform cross section comprising a major portion and a minorportion weakly connected together lengthwise thereof, said weakconnection between said major and minor portions being sufficientlystrong for use in suspending said major portion from said minor portionWhile concrete mix is being placed about said major portion to embed thelatter in concrete mix to a depth such that said weak joint lies in aplane generally flush with the surface of said concrete mix and whilesaid mix is taking a set, said weak connection being thereafter easilyseverable to disconnect the minor portion from the major portion of saidstrip leaving the latter embedded in and bonded to said concrete andexposed only along the portion thereof lying in a plane substantiallyflush with the finished surface of the concrete.

2. An article of manufacture as defined in claim 1 characterized in thatsaid strip of plastic material includes at least one web portiongenerally normal to the exposed edge surface of said strip when embeddedin concrete and having other web portions projecting laterally from saidfirst mentioned Web portion.

3. An article of manufacture as defined in claim 1 characterized in thatsaid major portion has voids therein out of communication with thelateral side wall areas of said strip and sealed against the entry ofmoisture and components of concrete mix while the latter is taking aset.

4. An article of manufacture comprising a continuous strip of semi-rigidmaterial for embedment crosswise of one face of a monolithic slab ofconcrete to separate said one face into adjacent surface areas, saidarticle being semi-rigid resilient homogeneous plastic material andhaving the same cross-sectional area from end to end thereof, and saidarticle including a main body portion and a relatively small sectionextending along and Weakly joined to said main body portion of saidstrip and adapted for use in suspending the main body portion of thestrip generally horizontally with said weak joint lying generally flushwith said one face of a concrete slab wtule the concrete mix is beingplaced about the suspended strip and taking a set, and said relativelysmall section adapted to be easily separable generally flush With saidone face of the slab after the mix has taken a set without disturbingthe bond between said concrete and embedded sur faces of said strip.

5. That method of laying a continuous monolithic strip of concrete alonga supporting surface having long narrow weakened zones of preselectedintervals which method comprises driving stakes into the ground in linesextending across the strip to be concreted, supporting a rigid member onsaid stakes with its lower edge no lower than contemplated for the topsurface of the concrete strip, suspending a noncompressed nonabsorbentstrip of unitary homogeneous resilient plastic material from said rigidmember and extending from edge to edge of said strip of concrete,pouring concrete mix in a continuous monolithic strip beneath and aboutsaid strip between the opposite ends thereof but leaving the top surfaceexposed, removing said stakes and said rigid member after the concretehas taken a partial set, and filling the stake holes with concrete mixflush with the upper surface of the concrete.

6. That method defined in claim 5 characterized in that said strip ofresilient material includes a weakened connection of reducedcross-section extending lengthwise thereof, suspending said strip abovethe subgrade so that said weakened connection lies substantially in theplane of the upper surface of the strip of concrete as the latter ispoured about the lower edge of said resilient strip, and removing theportion of said resilient strip along said weakened connection after theconcrete mix has taken at least a partial set leaving the remaininglower portion of said resilient strip embedded in the concrete.

7. That method defined in claim 5 characterized in the use of plasticfoam for said strip of resilient material, and further characterized inthe step of removing said strip material after the concrete mix hastaken a set by applying a solvent to the exposed surfaces of said stripeffective to dissolve said plastic foam material.

8. That method defined in claim 5 characterized in the step of applyinga liquid solvent to the exposed edge of said strip of resilient materialafter said concrete mix has taken at least a partial set whereby saidstrip is dissolved leaving a channel extending crosswise of saidcontinuous strip of concrete.

9. That method of constructing concrete pavement having resilientseparator members embedded therein with the upper edge thereof generallyflush with the surface of the finished pavement, said method comprisingsuspending elongated resilient unitary separator members of homogeneousthermoplastic material having a plurality of flanges lying generally atright angles to one another lengthwise of said separators across thearea to be paved, said separators being suspended from above with anedge of the uppermost flange generally flush with the intended outerface of the pavement being laid and with all remaining portions of allof said flanges positioned spaced substantially above the pavementsubgrade for embedment and direct contact with concrete to either sideand below said separator members, filling the area to be paved withconcrete mix, and leveling the mix to lie flush with the uppermost edgeof the upper flange of said separator members thereby leavingsubstantially all remaining portions of said flanges in direct contactwith said concrete mix from end to end thereof.

10. That method defined in claim 9 characterized in that said separatormembers are generally T-shaped in cross-section and in that said membersare so supported prior to the pouring of said concrete mix that one ofsaid flanges lies in a generally vertical plane and that another flangeprojects laterally therefrom in a generally horizontal plane spacedsubstantially above the pavement subgrade, said vertically andhorizontally disposed flanges cooperating with one another in anchoringthe separator members rigidly within the concrete pavement and insealing the finished pavement against entry of moisture along theinterface between the concrete and the surfaces of said separatormembers.

11. That method of constructing monolithic concrete pavement havingembedded therein resilient separator members extending only partiallythrough the thickness of said pavement, said method comprising providingelongated separator strips of resilient homogeneous nonmetallic materialhaving a major portion adapted to be embedded within and bonded to saidpavement and a relatively small portion thereof weakly joined to saidmajor portion and lying outside the surface of said pavement for use insupporting said major portion while concrete mix is being placedthereabout and While said concrete mix is taking a set, utilizing saidsmall portion of said strip to hold the strip as a whole suspended abovethe pavement subgrade, placing concrete mix over the suhgrade to a levelflush with the weak joint lengthwise of said separator strip, allowingsaid poured mix to take a set, and thereafter removing the exposedportion of said strip along said weakly joined area close to the surfaceof the finished concrete pavement.

References Cited by the Examiner UNITED STATES PATENTS 1,508,545 9/1924Foster 94-51 1,516,100 11/1924 Hubbard 94-51 1,562,257 11/1925 Rodgers94-51 Galassi 94-17 Heltzel 94-51 Robinson 50-70 Older 94-17 Thomas etal 94-18 X Fischer 94-18.2 Van London 94-18 X Kelley 94-17 Bartholow etal. 94-18 Carnes 94-17 X Maude 94-18 Worson 94-18 Middlestadt 94-18Crone 94-18 JACOB L. NACKENOFF, Primary Examiner.

CHARLES E. OCONNELL, Examiner.

N. C. BYERS, Assistant Examiner.

1. AN ARTICLE OF MANUFACTURE COMPRISING A STRIP OF HOMOGENEOUS PLASTICMATERIAL OF UNIFORM CROSS SECTION COMPRISING A MAJOR PORTION AND A MINORPORTION WEAKLY CONNECTED TOGETHER LENGTHWISE THEREOF SAID WEAKCONNECTION BETWEEN SAID MAJOR AND MINOR PORTIONS BEING SUFFICIENTLYSTRONG FOR USE IN SUSPENDING SAID MAJOR PORTION FROM SAID MINOR PORTIONWHILE CONTRETE MIX IS BEING PLACED ABOUT SAID MAJOR PORTION TO EMBED THELATTER IN CONCRETE MIX TO A DEPTH SUCH THAT SAID WEAK JOINT LIES IN APLANE GENERALLY FLUSH WITH THE SURFACE OF SAID CONCRETE MIX AND WHILESAID MIX IS TAKING A SET, SAID WEAK CONNECTION BEING THEREAFTER EASILYSEVERABLE TO DISCONNECT THE MINOR PORTION FROM THE MAJOR PORTION OF SAIDSTRIP LEAVING THE LATTER EMBEDDED IN AND BONDED TO SAID CONCRETE ANDEXPOSED ONLY ALONG THE PORTION THEREOF LYING IN A PLANE SUBSTANTIALLYFLUSH WITH THE FINISHED SURFACE OF THE CONCRETE.